1. Field of the Invention
The present invention relates generally to calibration devices. More particularly, the invention concerns a device for the calibration of patient dose measurement and control systems of the character used in diagnostic radiology.
2. Discussion of the Prior Art
It is well known that overexposure of a patient to X-ray radiation can have serious adverse effects and should be minimized. Accordingly, it has long been recognized that some type of system for measuring the dose of X-ray radiation received by the patient is needed. In the past, dose-area product (DAP) has been measured in a number of different ways to evaluate the radiation exposure of the patient. The dose-area product of an X-ray beam may be defined as the surface integral of absorbed dose to air over a plane perpendicular to the beam.
Several prior art systems and devices, including DAP meters, have been proposed to measure DAP during diagnostic radiological procedures. In making these measurements, a full-field ionization chamber is typically mounted on the collimator face of the host equipment to measure the product of the air kerma or patient surface dose multiplied by the area of the radiation field. This measured dose area product, or KAP (kerma area-product) has the novel advantage of being a constant irrespective of distance from the radiation source.
The DAP, in combination with information on X-ray field size can be used to determine the average dose produced by the X-ray beam at any distance downstream in the X-ray beam from the location of the ionization chamber. DAP meters are usually calibrated approximating the integral by the product of the field area and the dose measured in the center of the field. However, the accuracy of this simplified method can be inadequate, especially when the meters are used for optimizing radiological procedures.
An alternative means of obtaining the dose area product is by calculation based on stored tables having data related to dose output at different X-ray generator kVp (kilo Voltage peak) and mAs (milli Amp seconds) settings. This data is multiplied by the radiation field area that is derived from collimator position sensing transducers.
Dose as opposed to dose area product is of greater interest when assessing local risk to organs including the skin as opposed to total body radiation burden. Dose can be obtained by mounting sensors on the patient's skin. However, this is generally considered to be excessively time consuming and inconvenient for routine use. An alternative approach is to measure the dose at the face of the collimator or calculate it based on machine settings and making corrections for the inverse square law relationship between doses measured at different distances from the radiation source.
In addition to the measurement of patient dose, it is increasingly common to control radiation exposure automatically. In this approach, a sensor is placed immediately in front of or to the rear of the image receptor, or dose related data is obtained directly from the image receptor output. In the past the objective of such devices was to optimize image quality. However, the emphasis is now directed toward the use of automatic control to ensure the dose used to produce an image is within appropriate reference levels.
At the present time, the common practice is for all equipment used for patient dose measurement and control to be calibrated during manufacture. This approach requires that the calibration be verified during equipment acceptance testing and at regular intervals during the life of host equipment. This calibration should ideally be referenced to the anticipated patient location and incorporate the influence of all objects in the beam-path including the table and the phantom that simulates the patient.
The current methods used to carry out such calibration checks involve measurement of central axis dose only and assessment of the field size based on image measurement. In addition to being extremely time consuming, such an approach is prone to both systemic and operator error.
As will better be appreciated from the discussion that follows the thrust of the present invention is to overcome the drawbacks of the prior art methods for carrying out calibration checks by providing a novel ion chamber having unique features that greatly simplify the testing and calibration of patient dose measurement and control devices.